A hydraulic tensioner typically includes a check valve for allowing oil to flow under pressure through an oil supply passage into an oil chamber formed by a tensioner housing and a plunger protruding from the housing, and for restricting reverse flow of oil from the oil chamber to the oil supply passage.
The check valve is a unit that includes a ball seat having an oil passage, a check ball that opens and closes the oil passage by separating from and seating on a surface of the ball seat, and a retainer that forms a valve chamber accommodating the check ball and restricting the stroke of the check ball, i.e., the distance through which the check ball moves when it separates from the ball seat. A typical check valve unit is described in the Japanese Patent Application laid open on Feb. 3, 1998 under No. H10-30695.
When the retainer and the ball seat of the check valve unit are composed of different parts, it is necessary to hold the check ball between the retainer and the ball seat during assembly of the check valve unit. Consequently assembly and installation of the check valve unit are difficult, and the cost of the check valve unit and the overall cost of the tensioner are increased.
The attitude of the tensioner depends on the manner in which it is installed. Accordingly, the angle between the center line of the ball seat in the check valve of the tensioner and the vertical direction can vary. The check ball of course tends to move downward within the valve chamber due to gravity. If a hole in the retainer that connects the valve chamber with the oil chamber of the tensioner is located directly underneath the check ball, the check ball tends to become displaced downwardly when the check valve opens, and it becomes difficult to form an oil film between the check ball and the wall of the valve chamber. As a result, the flow of oil into the hydraulic chamber varies considerably due to retardation of the closure of the check ball against the valve seat, and the check ball tends to move in a disorderly manner. The result is an impairment of the performance of the tensioner. Setting back, i.e., retraction of the plunger, can be impaired by premature closure of the check valve. Alternatively, slow flow of oil through the check valve can delay the advancing movement of the plunger and thereby delay the application of tension to the traveling transmission medium.
Accordingly, there is a need for a hydraulic tensioner that can be assembled more efficiently and at a reduced cost. There is also a need for a hydraulic tensioner in which the circumferential positioning of the check valve unit is independent of the attitude of the tensioner so that the passage or passages leading from the valve chamber of the check valve to the oil chamber of the tensioner are automatically positioned optimally for suppression of disorderly movement of the check ball. There is a further need for a hydraulic tensioner in which the application of tension is quick and stable, in which movement of the plunger in the setback direction is well-controlled.